In humans, deficiency of galactose-1-phosphate uridyltransferase (GALT) caused by mutations in the GALT gene leads to a potentially lethal disease, classic galactosemia. If galactose is not withdrawn from the diet of affected infants within five to nince days of life, some newborns die of hepatic failure and Escherichia coli sepsis. Although all 50 states in the U.S. include this metabolic disorder in newborn screening programs and a galactose-restricted diet prevents neonatal death in some affected infants, many surviving patients develop debilitating complications including growth restriction, premature ovarian failure, speech dyspraxia, ataxia, and other neurological deficits. The long-term goal of this project is to develop more effective therapies for patients with classic galactosemia. Earlier, the investigators defined the structure-function basis of specific GALT mutations and established genotype-phenotype relationships for disease severity and outcome. Using patient cells that are homozygous for the Q188R-GALT mutation, they recently showed that galactose insult to these cells resulted in accumulation of galactose-1-phosphate (Gal-1-P) and manifestation of endoplasmic reticulum (ER) stress prior to cell death. Through serendipity, they also discovered that supplementation of specific antioxidants protected these cells from galactose toxicity. They hypothesize that patient genotypes, which dictate the amount of Gal-1-P accumulated, define the degree of cellular toxicity via mechanisms involving ER stress and oxidative stress. To test this hypothesis, the investigators propose, in Specific Aim 1, to derive diploid fibroblast cells strains from galactosemic patients homozygous for three different mutant alleles: Q188R, S135L, and ?5kb. They will characterize these cell strains and use them to test the efficacies of novel therapies;in Specific Aim 2, to discover small molecule inhibitors for human galactokinase (GALK), the enzyme responsible for the production of Gal-1-P, and to test their efficacy in preventing ER stress in GALT-deficient cells stratified by genotypes;and in Specific Aim 3, to delineate the molecular mechanisms by which antioxidant supplementation protects GALT-deficient cells from galactose toxicity, stratified by genotype.